Semiconductor devices designed to operate with high currents usually comprise a wafer of a semiconducting material, often silicon, which is contacted on at least one side with an intermediate plate of a material (e.g. molybdenum) having approximately the same coefficient of thermal expansion as the semiconducting material.
A semiconductor device is further provided with electrical connecting bodies, usually of copper, for the supply and discharge of electric current to and from the semiconductor wafer. The connecting bodies are often arranged in direct pressure contact with the semiconductor wafer. They are normally connected to cooling bodies, usually of copper or aluminium, so that the semiconductor wafer is cooled from one or both directions. The connecting bodies themselves may also be designed so as to serve as cooling bodies.
The role of an intermediate plate is to protect the semiconductor wafer from stresses caused by mutual movements between the semiconductor wafer and the connecting bodies upon temperature variations caused by differences in the coefficients of thermal expansion. To obtain an efficient discharge of heat from the semiconductor wafer, the intermediate plate needs to have high thermal conductivity.
Molybdenum and tungsten are examples of materials which have the required thermal conductivity and coefficient of thermal expansion for an intermediate plate but both are extremely expensive components in semiconductor devices.
One object of the present invention is to provide a cheaper material than molybdenum, which preferably has approximately the same coefficient of thermal expansion as molybdenum, or at least a substantially lower coefficient of thermal expansion than copper, and which, in addition, has a sufficiently high thermal conductivity to replace molybdenum as the material for an intermediate plate.